Scrollbar in slide mode

There is `overflow-y: scroll`defined [right here in the CSS](https://github.com/hackmdio/codimd/blob/e89f8205426b2f93e3127b28cd5902cb6f5b4426/public/css/slide.css#L325) that makes the slide mode always show a vertical scrollbar. This is a distraction for presentations, when the browser is in full screen mode and the dark slide background should merge seamlessly with the surroundings.


It looks fine if this rule is removed:


Before going for a pull request, I would like to ask whether this has been added on purpose, because it is obviously [not part of reveal.js](https://github.com/hakimel/reveal.js/search?q=overflow-y).